A variety of automatic film processors are available which are capable of developing films exposed to various forms of energy. The exposure of the film to that energy creates a latent image on the film. In order to render that image visible, the film is passed through a developer solution. In the automatic film processors referred to above, development takes place in a developer tank. During processing, the film passes through the developer tank where the latent image becomes visible. The film is then passed through a tank holding fixer solution which hardens and preserves the visible image. The film is then washed and dried by the processor so that it is available for analysis. An example of such a system is shown in U.S. Pat. No. 4,650,808 to Burbury which is incorporated into an automatically self-cleaning processor.
The crucial step in this automatic film processing is the development step. Several variables can affect the quality of development. Included among these are the temperature of the developer solution, and the amount of active developer solution that reaches the film. In order to continually provide the film surface with active developer solution, it is desirable to continually agitate the solution. In prior art systems, both agitation and temperature control of the developer are achieved by means of a developer recirculation system. A typical recirculation system is shown in FIG. 1. The recirculation system RS is connected between the drain D and inlet I of the developer tank T. Developer is recirculated through the system by means of pump P. Developer solution drawn out of tank T though drain D is pumped by pump P through a heater H and back into the tank through inlet I. A heat sensor S is connected to heater H. The thermostat TH receives a temperature signal from the sensor S, and selectively activates and deactivates the heater H to maintain the developer solution at an optimum temperature, illustratively 93.degree. F. While maintaining the proper temperature for the developer, the recirculation RS system also agitates the developer. Even with such agitation, however, a given quantity of developer solution has a limited effective lifetime, and the developer solution must be either changed or replenished periodically.
Other prior art systems are available which include an automatic replenishment system for the developer solution. Typically, such a system includes a replenishment tank and a separate pump for pumping replenishment solution to the developer tank. The replenishment system is used in conjunction with the recirculation system described above. Such a system is shown in FIG. 2. The replenishment pump P.sub.2 pulls replenishment developer out of the replenishment tank RT, and adds it to the developer tank at inlet I.sub.2, causing the level of developer in the developer tank to rise. The developer tank includes an overflow outlet O, from which the overflow may be returned to the replenishment tank RT. At the same time, the operation of the recirculation system progresses as described above as driven by pump P.sub.1. In effect, then, the recirculation system is expanded to include the contents of the replenishment tank. These systems are generally referred to as closed-loop systems. Since replenishment tanks may have a volume greater than that of the developer tank, closed-loop systems can effectively more than double the developer available to the automatic processor. The increased volume of developer thus has a longer effective lifetime, since that larger quantity of developer becomes contaminated more slowly than in the previously-described system.
While such systems do prolong the lifetime of the developer solution by slowing contamination, they are not without significant drawbacks. Since both the contents of the developer tank and the contents of the replenishment tank are continually used for development, that entire volume must be heated to the optimum temperature. As a result, the heater must heat not only the developer being recirculated to the developer tank, but also the entire contents of the replenishment tank, which may be significant illustratively being on the order of 5 gallons. This means that a higher-power heater must be used as compared to systems wherein the heater just heats the recirculating developer. Furthermore, the warm-up time of such an automatic film processor is significant. Long warm-up times can be a serious drawback in such system, particularly in environments where the processor is only used periodically. In such environments, the processors typically go into a standby mode between processing cycles. In the standby mode, the recirculation pump is not pumping, and the heater is not activated. Thus, when the processor comes out of standby mode, the developer must be re-heated before a developing cycle can occur. When the processor is only used periodically, it is inconvenient and time-consuming to wait for re-heating of the increased volume of developer before each cycle. In systems where the processor is being used continually, this long warm-up time occurs only when the processor is first turned on. However, the higher-power heater consumes large amounts of electricity in maintaining the increased developer volume at the optimum temperature. The temperature control in such system is also imprecise. Since the replenishment tank has such a large volume, some of the developer solution remains in the tank for significant periods of time before being recirculated to the developer tank. During this time, this developer may cool. As a result, cool developer solution is added to the recirculation system, thus potentially creating large temperature fluctuations in the developer which adversely affect the quality of development.
In other prior art systems, the overflow from the developer tank is applied to an external drain as opposed to being returned to the replenishment tank. Such a configuration is shown in phantom in FIG. 2. In this arrangement, the contents of the recirculation tank RT are not effectively added to the recirculation system. Rather, the cold developer from RT is periodically added to the developer tank and the tank overflow goes to the external drain. The addition of replenishment developer to the developer tank typically only occurs when film is in the processor, as continual replenishment would result in undue waste. Since only the contents of the recirculation system is being heated in this configuration, initial warm-up times are lower than in closed-loop systems. However, cold developer is periodically added directly to the developer tank during development. This adversely affects the temperature of the developer solution and thus development quality.